1. Field of the Invention
The present invention relates to a line switching apparatus, more particularly to a switching request detection circuit incorporated therein for detecting a switching request signal output by a line in which a fault has occurred.
2. Description of the Related Art
In a multiplex data transmission system, a protection line is provided for a plurality of N main lines, i.e., normal working line, for example, eight lines. When a fault occurs in one of the main lines, a transmission on the faulty main line is switched to the protection line, to maintain the inherent data transmission. This is known as an N vs 1 line switching apparatus. In such a line switching apparatus, there is a current trend toward an increase of the number of channels in accordance with the increased amount of data to be transmitted, and accordingly, the amount of information on the switching request signal is also increased. The switching request signal (hereinafter switching trigger) is generated in response to a line fault or restoration of a faulty line.
In the detection of the switching trigger, a guard time must be taken into consideration, to prevent an erroneous line switching due to an instantaneous pulse induced by noise and the like. But although the guard time is useful for preventing the above-mentioned erroneous line switching, it is disadvantageous from the viewpoint of the limitation of a prescribed line switching processing term, for example, less than 50 ms, imposed thereby.
In the prior art switching trigger detection circuit, the line switching decision is performed by software using a microprocessor, as will be explained later. Therefore, when the number of switching triggers increases, a heavy load due to the related processing is imposed on the microprocessor, and thus the microprocessor controlled switching trigger detection circuit has a problem in that often a correct line switching cannot be completed within the aforesaid prescribed short term, since the microprocessor requires a relatively long time to process and output each line switching decision.